This invention relates to a device for ejecting a chemical reactant into a large scale combustion device such as a boiler for reaction with flue gases.
In an effort to reduce NO.sub.x discharge from large scale combustion devices such as boilers burning fossil fuels, selective non-catalytic reduction (SNCR) is provided by injecting ammonia or other chemicals such as aqueous urea into the flue gases downstream of the combustion zone. By reacting ammonia (NH.sub.3) with Nox; nitrogen, water and other products of the chemical reaction are generated thus reducing the environmental impact of the emissions from the combustion device. There are also instances in which it is desired to introduce other chemical reactants into a flue gas stream of a large scale combustion device.
Ammonia and other chemical reactants may be introduced into flue gases using various approaches. In one approach which is commonly used, nozzles positioned on the wall of the boiler spray a chemical reactant into the flue gases. Although this approach produces beneficial results, it has the disadvantage that distribution of the chemical reactant is non-uniform across the high temperature flue gas stream. Another approach for chemical injection is to employ an elongated ejection device which transverses the flue gas stream and dispenses a chemical reactant through nozzles at various positions. One of the principal difficulties in providing such a transversely extending distribution device is related to the high temperature corrosive environment present in the boiler. This environment makes it difficult to provide devices that will operate reliably over a long period of time and are at the same time cost efficient. Typical metal alloys corrode in such an atmosphere and undergo structural degradation in response to the high temperature exposure.
When employing SNCR emission control it is important to inject the chemical reactant into the boiler flue gas when the flue gas is within a particular temperature range. The optimum injection location in the boiler however, varies with load and fuel characteristics. As a result, it would be desirable to provide injection devices at multiple locations which can be actuated as needed based on the temperature range of flue gases in a given area.
When discharging a chemical reactant from a device extending transversely through the flue gas field it is desirable to inject the chemical reactant at various positions in a repeatable uniform manner. Accordingly, some means of insuring homogeneous mixing of the chemical reactant and the carrier gas used is a desirable attribute of such an injection system.
Another desirable feature of a retractable chemical injection device would be the ability to change the angle at which the injected chemical is introduced into the flue gas stream. In order to achieve optimal distribution of the chemical reactant in the fluid stream, it may be necessary to orient the spray at a particular angle with respect to the flow stream. This angle may not be predictable precisely in advance and therefore, experimental or automatic adjustment of the unit in service may be required.
A further desirable feature of such a device would be the ability to implement presently available mechanisms used in the large scale boiler industry, thereby using existing infrastructure for fabrication, operation and maintenance.
The above mentioned desirable attributes of a chemical reactant injection system are provided in accordance with the devices of the present invention.
In accordance with the present invention, a chemical injection device is provided having a retractable lance tube which can be extended into and retracted from the flue gas stream. The lance tube slidably overfits a stationary feed tube. A plurality of discharge nozzles are distributed axially along the lance tube. At the proximal end of the feed tube, a supply of compressed air or another injection carrier such as steam is delivered. One or more atomizer nozzles is positioned within the feed tube and is provided to introduce the chemical reactant which is generally an aqueous solution. By spraying the reactant into the air or steam injection carrier flowing through the feed tube, the reactant becomes entrained within the injection carrier as it passes from the feed tube into the lance tube which provides a number of benefits. First, the latent heat of evaporation of the reactant helps in absorbing heat from the lance tube thereby cooling the lance tube. Moreover, the evaporation of the reactant droplets provides a finally atomized and thoroughly mixed fluid, enabling a uniform distribution of the chemical reactant from the distributed nozzles.
Further refinements of the devices according to this invention include coating the outer surface of the lance tube with a material which lowers its emissivity, thereby reducing radiative heat transfer to the lance tube which allows it to operate at a reduced temperature. Another enhancement is the incorporation of a rotary drive mechanism which allows the angular position of the lance tube, and consequently the discharge nozzles to be changed as desired in use.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.